1. Field of the Invention
The present invention relates to an objective lens, a reproducing apparatus and a reproducing method using this objective lens which is arranged so that a light beam emitted from a common light source may be focused through a single lens on respective signal recorded surfaces of at least two optical disks having light transmitting layers of different thickness for selectively reproducing the optical disk.
2. Background of the Invention
In the past, the compact disk (hereinafter referred to as a CD) has been widely spread as the optical disk which is a recording medium for reproducing recorded information using the light beam. Recently, however, new recording media for digitally reproducing a long video record or the like such as the digital video (Versatile) disk (hereinafter referred to as a DVD) has been developed.
When digital information is to be read out of the recording medium using the light beam as described above, a laser light beam is irradiated at pits on tracks of the recording medium and its reflected light is detected. A level of the reflected light is calculated and converted to binary data by a reproductive processing unit and the like, thereby enabling recorded information to be reproduced.
FIG. 1 shows an example of a structure of an optical head unit for a CD. A laser diode 21 forming a light source emits a laser light whose wave length is 780 nm. A grating 22 is arranged to divide one laser light emitted from the laser diode 21 into a plurality of (substantially three) laser lights, one of these three laser lights being used for readout of data a focusing servomechanism, and two remaining laser lights being used for a tracking servomechanism.
A beam splitter 23 which is a spectroscopic means made of transparent parallel plates reflects toward an objective lens 24 the laser light which is emitted from the laser diode 21 and is incident thereon through the grating 22. The beam splitter further gives an astigmatism to a reflected (i.e., or converged) light by a CD 100 which has returned through the objective lens 24 and makes that light to penetrate therethrough to a photodiode 25.
The objective lens 24 converges the laser light and makes it irradiate on an information recorded layer 102 of the CD 100 where fine pits are arranged. The objective lens 24 further converges the reflected light from the information recorded layer 102 of the CD 100 and makes it incident on the photodiode 25 through the beam splitter 23.
In addition, concerning the objective lens 24, as its numerical aperture (NA) becomes greater, the converging angle of light becomes larger, thereby allowing the light to be converged on a smaller area. In this example, an objective lens having the NA of 0.45 is utilized as the objective lens 24.
The photodiode 25 is arranged to detect the returned light of the laser light which is made to irradiate on the CD 100 by the laser diode 21. Since the laser light is divided into three lights by the grating 22, the photodiode 25 has also three light receiving parts corresponding thereto. One of these parts of the photodiode 25 is a light-receiving one for receiving the laser light which has read data. Two remaining two parts thereof are utilized to receive the two laser lights for tracking and to control the tracking of the objective lens 24 on the basis of an amount of the received lights so that the laser light for reading data may be irradiated on a predetermined track.
The laser light reflected by the information recorded layer 102 and then being incident on the photodiode 25 involves an astigmatism because it passes the beam splitter 23 as a converging light. The focusing servo operation will be effected by using this astigmatism.
The CD 100 as a recording medium comprises the information recorded layer 102 formed on a substrate 101 which is a light transmitting layer having a thickness t of 1.2 mm and a protecting film 103 formed on the information recorded layer 102. The laser light emitted from the laser diode 21 penetrates the transparent substrate 101 and then irradiates the information recorded layer 102. The information recorded layer 102 has pits corresponding to the recorded information. When the laser light irradiates the pits, the diffraction is caused so that an intensity of the returned light (i.e., the light reflected on the recording medium and then being incident on the photodiode 25) will be weakened. When the laser light irradiates a portion with no pit, it is intactly reflected so that the intensity of the returned light will be strengthened. The photodiode 25 detects such returned lights and converts the intensity of the returned lights to binary digits "1", "0". Thus, the data recorded on the CD 100 as the pits are read out.
In the way described above, the recorded information is read out by making the laser light irradiate the pits of the CD while performing the tracking servo and the focusing servo, and then detecting its returned light.
Recently, on the other hand, a DVD 200 with such a structure as is shown in FIG. 2 has been proposed. While the CD 100 has an information recorded on its only one side surfaces, DVD 200 has information recorded on its both side surfaces. Particularly, the DVD 200 is made of two parts in one part of which an information recorded layer 202 is formed on a substrate 201 and a protecting film 203 is formed on the information recorded layer 202, and in the other portion of which an information recorded layer 302 is formed on a substrate 301 and a protecting film 303 is formed on the information recorded layer 302. The two parts are secured by joining both the protecting films 203 and 303.
In the DVD 200, since information is recorded in high density, the substrates 201 and 301 are made smaller as compared with that of the CD 100 in thickness in order to alleviate an influence due to a skew, an error of the substrate's thickness, and so forth. In other words, the substrate 101 of the CD 100 is made to be 1.2 mm thick, whereas the substrates 201 and 301 of the DVD 200 are made to be 0.6 mm thick, respectively. Also, a pit length and an interval between pits in the DVD 200 are made smaller than those of the CD 100.
Because the recording density of such DVD 200 is thus higher than the recording density of the CD 100, a laser diode which generates a laser light having a shorter wave length (635 to 650 nm) than that of the laser diode 21 for the CD is employed as the laser diode 41 of the optical head unit for the DVD. Moreover, a grating 42, a beam splitter 43, an objective lens 44 and a photodiode 45 other than the laser diode have the same structure as that of the optical head unit for the CD.
However, because the DVD 200 has a minute pit as compared with that of the CD 100, an objective lens having a numerical aperture NA which is larger (NA=0.6) than that (NA=0.45) of the objective lens 24 for the CD 100 is used as the objective lens 44 for the DVD 200. By using such objective lens 44 whose numerical aperture is large, it is possible to converge the laser light on a smaller area for reading the minute pits.
As described above, since the structure of the objective lens differs between the CD 100 and the DVD 200, it is normally required to employ different optical head units in order to read the information out of the recording medium. For example, when the optical head unit for the DVD is to be utilized for the CD, an influence by the spherical aberration due to a difference between thickness of the substrates 101 of the CD 100 and those of the substrates 201 and 301 of the DVD 200 and a difference between the numerical apertures will occur.
For example, when the reproduction is performed on a CD having the substrate whose thickness is 1.2 mm through the objective lens with the numerical aperture of 0.6 which is optimized for the DVD 200 having the substrate whose thickness is 0.6 mm, the spherical aberration occurred amounts even to 3.6 .mu.m in the fourth order Seidel spherical aberration coefficient W.sub.40. Expressing this by the root mean square (rms), it will be 0.268 rms.multidot..mu.s (if normalized by wave length .lambda. (=650 nm), 0.412 rms .multidot..lambda.). For such reason, it is generally required in the optical disk that the total sum of the root mean square of the aberrations of all optical systems is less than the Marechal criterion value 0.07 rms.multidot..lambda.. Therefore, it is difficult to read data from the CD 100 with the optical head unit for the DVD.
Thus, the applicant of the present invention has proposed, for example, in patent application Ser. No. 277400/1994 that the optical head unit for the DVD can be applied to the CD by adjusting the NA of the objective lens depending on a sort of the recording medium.
FIG. 3 and FIG. 4 illustrate the principle of what is described above. As is shown in FIGS. 3 and 4, this structure further includes a diaphragm 51, a driver unit 53 for driving this diaphragm 51 and a discriminating device 52 for discriminating the sort of the recording medium in addition to the structure of the optical head unit for the DVD as shown in FIG. 16.
When the discriminating device 52 discriminates the sort of the recording medium, and then the driver unit 53 works to read the data from the DVD 200 correspondingly to a result of the discrimination, it makes the diaphragm 51 widely open so that the NA of the objective lens 44 may be 0.6 as shown in FIG. 3. In constant to this, when reading the data from the CD 100, the driver 53 makes the diaphragm 51 less widely open so that the NA of the objective lens 44 may be 0.45 as shown in FIG. 4. In other words, when reading the data from the CD 100, it is possible to perform the readout operation by reducing the numerical aperture of the objective lens 44 for alleviating the influence of the spherical aberration (W.sub.40 is proportional to the fourth power of the numerical aperture).
However, if such diaphragm 51 as described above is newly provided, the number of parts of the optical head unit will then increase to make it expensive and a scale of the whole apparatus will also become bulky. Furthermore, since the diaphragm 51 is mechanically operated, it tends to delicately be affected by vibration. Besides, it is hard to act promptly, which raises a problem that it tends to cause a trouble.
As an alternative means though not shown, an optical system has been proposed, in which the readout of a CD is also ensured by combining a hologram lens with the objective lens. In this case, however, there is a problem that adding the hologram lens makes the cost increase and in particular it will be very difficult to control the aberration of the optical system.